The invention relates to a magnet system with superconducting field coils for generating a constant magnetic field, suitable for NMR experiments, which has a high field strength in the experimental volume and sufficient homogeneity for NMR tomographic and/or spectroscopic experiments.
Such magnet systems, which have two coil systems of different mean diameters arranged coaxially with respect to the longitudinal axis defined by the field direction in the homogeneity region, which are rotationally symmetrical and are arranged symmetrically about the transverse median plane perpendicular to the longitudinal axis, and which can be excited with currents which generate magnetic fields that compensate, at least approximately, for the dipole fields of the two coil systems in the outer space of the magnet system, are known (for example from EP 0 138 270 A2, EP0 144 171 A1, DE 38 29 175 A1, and GB 2 206 242 A).
In the magnet system disclosed by EP 0 144 171 A1, the field coils whose (dipole) fields are intended to be largely compensated for in the outer space of the magnet system are permanently wired in series. In the magnet system disclosed by DE 38 29 175 A1, parallel wiring of the field coils which develop the two oppositely aligned magnetic moments is also disclosed as an alternative to series connection thereof. Although these known systems can be operated with currents of varying intensities without thereby significantly impairing the shielding effect in the outer space, the usable field strengths are nevertheless limited, by the fact that the magnetic field generated by the outer field coil is always aligned opposite to the magnetic field generated by the inner field coil, to a value that is less than the absolute field strength of the magnetic field generated by the inner field coil.
In contrast, the magnet systems of the aforesaid kind disclosed by EP 0 138 270 A2 and GB 2 206 242 A are configured in such a way that the two superconducting field coils can be energized with currents of different strengths, with at least one of the two field coils being energized inductively, and the two field coils each forming a self-contained current circuit. Depending on the current strength value to which the shield--i.e., outer--field coil is energized, it is then possible to vary the field strength that can be utilized for an experiment between a minimum value--which results when the outer, shielding field coil is energized to a maximum value for its operating current strength (assuming a predetermined charge of the inner field coil)--and a maximum value which results when the outer, shielding field coil is not energized.
The magnetic field strengths in the experimental volume that can be utilized with known magnet systems (i.e., those having active shielding provided by an external field coil in the outside space) are therefore two low for many experimental purposes, so that the known magnet systems appear to be suitable, at best, for tomographic experiments in which the primary requirement is good field homogeneity in the largest possible experimental volume, while high field strength is not of critical importance. However, the known magnet systems are therefore unsuitable for investigative purposes such as NMR spectroscopy experiments, in which less emphasis is placed on good homogeneity of the magnetic field in the largest possible experimental volume at moderate field strength, while the highest possible field strength--along with good field homogeneity in a relatively small experimental volume--is important.